Apparatus for separating suspensions of catalyst particles and reacted mixture of hydrocarbons and a catalytic cracking process

Abstract: The invention concerns apparatus for separating catalyst particles in the catalytic cracking of reacted hydrocarbons, comprising an unconfined cyclone device made up of a diplegless cyclone (4) opening directly into a large volume separator vessel (21), below by means of a mouth (19) and above by means of the annular space between concentric pipes (6,7; 30,31; 40,41; 26, 27) to the other components of the system. The invention also comprises the hydrocarbon fluid catalytic cracking process carried out in such apparatus.

Description: This invention relates to apparatus for separating solids from gas and to a process for the catalytic cracking of hydrocarbon feedstocks, whether or not high boiling point hydrocarbons are added. More particularly the invention is of apparatus used to separate, from a reacted hydrocarbon mixture, particles from a catalyst suspension in a catalytic cracking process. The novel and revolutionary idea of the apparatus enables the gas phase of particulate suspensions to be separated out more efficiently. The invention also relates to the operation of such an apparatus as well as to a new fluid catalytic cracking process (FCC) springing therefrom. In the fluid catalytic cracking process (FCC) the purpose is to convert high boiling point hydrocarbons into light hydrocarbon fractions such as liquified petroleum gas. The catalyst used in FCC is a very fine powder, particles of which act like a liquid when fluidised in steam or air. The fluidised FCC catalyst circulates continuously between the reaction and regeneration zones. In the first of these, together with the cracking reactions, a carbonaceous deposit (coke) is created on the surface of the catalyst, reducing the activity and selectivity of the catalyst. Removal of such deposit takes place in the second zone, by its being burnt in air, the activity and selectivity the of catalyst becoming high again. The catalyst also acts as a medium for the transfer of heat from the regenerating to the reacting zone. Upon introduction of catalytic cracking catalysts containing zeolites, particularly the ultra-stable zeolites, together with the use of high reaction temperatures and cracking with a short residence time in a riser reactor, fresh areas were found in which to develop the technique so as to enable advantage to be taken of the high activity and selectivity of such zeolitic catalysts. The usual technique consists of feeding the catalyst mixture, as a warm suspension, together with the sprayed hydrocarbon droplets into a riser where cracking reactions take place. Residence time for the reacting mixture is from 0.5 to 8 seconds in reaction temperatures of over 485 degrees Celsius. As mentioned, along with such cracking reactions, a harmful carbonaceous deposit (coke) develops on the surface of the catalyst which leads to a drop in activity and selectivity. After the riser it is particularly advisable that coked catalyst particles be swiftly separated from the cracked hydrocarbon suspension, in order to avoid any lengthy contact of gas and particle phases which would lead to the development of side reactions known as overcracking. Such undesirable overcracking reactions which convert noble products, as for instance gasoline, into fractions of heating gas, coke and liquified petroleum gas (LPG), are basically brought on by heat and take place due to lengthy contact time between the gas phase of the reacted mixture and the particulate solid phase of the catalyst, or merely because of an overlengthy residence of the gas phase of reacted mixture at a high temperature in the separation zone. In the usual technique the suspension of catalyst and cracked hydrocarbons from the riser is fed into the separating vessel, generally as a descending jet, where most of the catalyst is separated by gravity. Cracked hydrocarbons in stripping fluid entrain some of the catalyst flow into the upper part of the separating vessel, where cyclone separators bring about the particulate phase separation, and then finally the gas phase goes on to the product fractioning system. The catalyst separated in the cyclone drops into the dipleg of the cyclone, becoming a dense column of solids that flows into the stripper, after pressure between the base of the cyclone dipleg has been equalised with that of the outside environment. Under this well known operation the pressure inside the cyclone is always less than in the pressure vessel, the cyclone dipleg having to be sealed off whether by submerging it in the fluidized catalyst bed of the stripper or by use of some kind of sealing valve placed at its bottom end. In the lower part of the separating vessel a fluidized bed of spent catalyst develops, which is stripped with the aid of a stripping fluid. This stripping process brings about the removal of the reacted gas phase which takes up inter- and intraparticle spaces, and also of some adsorbed heavy hydrocarbons, thereby preventing them from being carried to the regenerator and thereby avoiding the unnecessary burning thereof, which would lead to a large rise in the temperature of the regenerator. In this usual way of carrying out the FCC process, the dimensions of separating vessel are large in order to provide for the riser end, disengagement room for solids, cyclone separators, and their respective diplegs, this leading to a large volume and therefore overlong residence time of reacted gas phase inside such vessel plus the aforesaid harmful effects brought about thereby. On the other hand so large a space can also be an advantage; for example: those engaged in such work will know only too well that risers do not operate in a uniform fashion; there may be a sudden rise in pressure, and/or in catalyst mass- and volume-flow rate, by as many as two to twenty times, brought about by changes in the operation of the unit, such as, for instance, the entrainment of an air pocket together with the hydrocarbon feed. Such variations are easily taken up by the great amount of room within the separator vessel without leading to any undesirable consequences such as entrainment of the catalyst into the fractioning system. In order to minimize reaction caused by any overcracking after the riser, brought about by the long time in contact with reacted gas and particulate solid stages, or merely due to residence of the reacted gas phase within the separator vessel, various methods and procedures have already been suggested. One of the most efficient among them is the system commonly known as the "closed cyclone" system which is based on the notion of the riser being directly linked to the cyclone separator. Along such lines there are many alternatives in the present state of the art: US-A-5171423 describes a large size external cyclone separator provided with a lower chamber fitted with baffles and a device for the injecting of stripping fluid which in turn feeds the reacted gas-phase suspension with some particulates to a separator vessel, where in the usual way cyclones bring about the final separation of the solid that has been entrained. The solid collected in the separator vessel flows into the cyclone separator by means of a pipeline for such purpose. Such external cyclone separator is meant to cut down on part of the charge of solids to be led into the separator vessel, and at the same time to begin the stripping process. The inventor of US-A-5171423 says that this arrangement is particularly useful for minimizing the effects of any discontinuous operation of the riser. In the preferred arrangement for the invention of US-A-5171423, the reacted gas stream that feeds the separator vessel is quenched by a cold stream of hydrocarbons in order to reduce temperature and to minimize the effect of any overcracking. US-A-4455220 describes a cyclone separator internally provided with a vortex stabilizer and a lower chamber for injecting the stripping fluid. In this device the catalyst, hydrocarbons and stripping fluid pass completely through the inside of the cyclone. The vortex breaking and ending device is meant to diminish the effects of the dragging of collected particles caused by entry of stripping fluid in the bottom part of the cyclone. EP-A-0545771 describes equipment much like that referred to above. The difference lies in the cyclone separator gas outlet goes downwards, enabling both feed and discharge gases to flow concurrently. US-A-4581205 describes the application of a small vessel between the cyclone and riser meant to accommodate pressure surges and flow surges arising out of any unsteady operation of the riser. This smaller volume vessel which fits into the reactor vessel is fitted with fluid injection to strip the catalyst in its bottom part. Side windows in the pipes connecting it to the riser, and in those of the smaller vessel itself and of the cyclone, enable any sudden expansion of gases to be dealt with. The top parts of these windows are hinged so as to enable them to open and to relieve the pressure. The stream of hydrocarbons and stripping fluid, together with some of the catalyst, flow from the smaller vessel into the cyclones. The separated catalyst flows along the diplegs of the cyclone which are fitted with check valves, and the gases flow into the fractioning system. US-A-4502947 provides a cyclone separator directly connected to the riser and to the first and second stage cyclones. Concentric pipes connect the riser cyclone gas outlet with the mouth of the first stage cyclone inlet. Stripping steam flows, together with some entrained catalyst, in the annular space between the concentric pipes. In this preferred configuration a pot of wider diameter than that of the riser cyclone leg, and lying in the bottom end thereof, allows the cyclone to be sealed off and enables catalyst gathered therein to overflow from it. Upon assembly and lining up of the concentric pipes it is suggested that different kinds of fillers be put inside the annular space while leaving some room for the stripping steam to flow within it. Another arrangement of this appliance, referred to in US-A-4623446, does away with the sealing pot in the riser cyclone leg thereby enabling stripping steam to flow through it, there being no need for the concentric pipes connecting the riser cyclone to the first stage cyclones. The size of the riser cyclone leg is dimensioned for operation at a speed of 0.03 to 0.30 meters per second, this being enough to minimize any catalyst entrainment into the cyclone, thus preventing any loss in efficiency. US-A-4588558 provides an alternative way of dealing with any sudden rise in pressure, by installing hinged windows in the pipe that connects the riser to the riser cyclone and in the interconnecting pipe to the cyclone first stage. Cyclone diplegs are fitted with check valves of the hinged type. Windows in the riser upstream of the cyclone connection provide a path for the stripping steam to flow from the separating vessel into the separation system. US-A-4961863 provides an alternative arrangement between the cyclone and riser in such a way that the axes of such equipment lie at right angles to one another. The curved surface of the cyclone thus lies at a tangent to the open upper end of the riser. The device is provided with a dipleg sealed off to the flow of any solids, and with at least one pipe lying on the same axis as the cyclone for the gas phase to flow. Stripping steam is injected into the cyclone, into the upper end of the dipleg that drains the particulate phase. Although progress has been made towards minimizing overcracking reactions in FCC processes nevertheless, in all the "closed cyclone" system alternatives referred to above, only cyclone separator devices featuring the confinement of the separated solid phase are provided in the riser outlet cyclone separating stage. In some instances the cyclone separators are provided with a dipleg to take the enclosed flow of the large mass of solids gathered, and likewise with means for sealing off the bottom part of the dipleg so as to avoid any loss in efficiency of the riser cyclone, caused by the flow of stripping fluid within it and consequent reentrainment of catalyst particles. In other instances the cyclone is the very vessel which encloses the stripping chamber, within which both separating and stripping take place, with the known collecting efficiency loss taking place in the cyclone separator. The use of enclosing cyclone separators makes it difficult to deal with any unsteady operation of a riser which leads to a drop in efficiency of the separator and therefore to undesirable overcracking reactions due to entrainment of the gas phase which reacted with the catalyst suspension, as well as to heavy catalyst losses to the product fractioning system and auxiliary equipment thereof. In trying to overcome this drawback, US-A-4478708 provides a method where the outflow of particles in suspension from a riser is separated by means of a cylindrical zone with its bottom part opened up, and its upper part connected peripherally to the riser by means of an enclosed radial path and connected tangentially to said cylindrical zone, which is closed at its upper end except for a coaxial pipe of small diameter along which the gas is withdrawn. Solids are discharged from the open part of the cylindrical zone. Separation takes place by centrifugal action; the enclosed feed paths for the cylindrical zone may be curved horizontally in order to get the centrifugal separation started/going. US-A-4666586 provides another method, like that of US-A-4478708 whereby separation takes place in one single zone which is shaped like an inverted cup. The major difference between these last two methods and those described above lies in the cyclone separating device connected directly to the riser and devised in such a way that there is no further need to confine the solids collected by means of a dipleg, that is, the cyclone is a non-confining cyclone not provided with a dipleg, its bottom half opening directly to the separator vessel, thereby taking advantage of the large volume of the separator vessel so as to take up any discontinuity in operation of the riser. Although helping to deal with the problem of controlling any unsteady operation of the riser encountered in the common closed cyclone systems, the non-confining cyclones disclosed in US-A-4478708 and US-A-4666586 are seriously handicapped by the fact that all of the stripping gas must pass through them, upwards and against the particulate flow, a fact which may lead to such particles being reentrained and consequently reducing efficiency. The present invention aims to provide separation apparatus specifically meant for use in FCC processes, even in those already in use, and which consists of an original and novel and low-cost idea suitable for such apparatus. The apparatus of this invention is characterised by the features of claim 1, and the process by the features of claim 6. The chief novelty of the system is that the cyclone-separating device is connected directly to the riser which comprises a cyclone, with no dipleg, that opens directly into the separator vessel, simultaneously in both the lower and upper parts, thereby achieving separation that is reasonably efficient and maintaining the gains derived from rapid separation of the reacted gas phase from the suspension of catalyst particles with its reduced activity and selectivity, as well as the gains due to dealing with the unsteady operation of the riser. A new FCC process is also disclosed herein, being brought about by use of such a separator apparatus, which is outstandingly better technically speaking, above all as regards the control over process variables. In order that the present invention may more readily be understood the following description is given, merely by way of example, with reference to the accompanying drawings, in which:- FIGURE 1 is a side view of the separator vessel employed in the fluid catalytic cracking process, and is one way of assembling the apparatus of this invention; FIGURES 2 and 3 are a side view and a cross-section, respectively, of one way of assembling the connecting pipe at the top part of device of this invention to the first stage of further separation; FIGURES 4 and 5 correspond to Figures 2 and 3 but show an alternative way of assembling the connecting pipe; and FIGURES 6 and 7 correspond to Figure 1 but show other alternatives of the system of this invention. The drawings form part of this specification but do not limit the invention in any way; they are merely meant to illustrate the invention and to make it easier to understand. Figure 1 serves to show that the apparatus of this invention consists of a device made up of a diplegless cyclone 4 directly connected to a riser 2 and also, by means of concentric pipes 6, 7, directly connected to a primary cyclone 8. It is associated with a fluid catalytic cracking process (FCC) for hydrocarbons, with or without added high-boiling hydrocarbons, which FCC process comprises intimately mixing a sprayed charge of hydrocarbons, in droplet form, together with a suspension of catalyst particles heated in a catalytic cracking zone 1, continuing cracking of said charge in the riser 2, feeding a considerably rich suspension of catalyst particles and cracked hydrocarbons directly into the separating device of this invention by way of a rectangular cross-section pipe 3 directly connected to the riser 2, bringing about the rapid separation of gas from particulate phases inside the diplegless cyclone 4, with the help of concentric pipes 6 and 7 feeding the gas stream containing some catalyst for later separation in the primary cyclone 8, and then by means of connection 9 feeding the gas to the secondary cyclone 10 from which the stream of gas substantially free of catalyst particles flows into the fractioning system along the outlet pipe 11 of the separator vessel 21. All of the catalyst separated out by cyclones 4, 8 and 10 is gathered in a small diameter vessel 5 which lies in the bottom part of separator vessel 21 from where it flows into the regeneration zone (not shown). In the separator vessel 21 the hydrocarbon gas phase is taken away (stripped) by inter- and intraparticle stripping, and part of some of the heavier hydrocarbons is adsorbed by countercurrently injecting stripping fluid into the descending stream of catalyst. In a preferred mode all of the stripping fluid together with the stripped matter joins the stream of cracked hydrocarbons which circulates in the diplegless cyclone 4 through its mouth 19. The catalyst separated out in both primary cyclone 8 and secondary cyclone 10, gathered in legs 16 and 15, becomes a column of solids which, after having reached the pressure needed for equilibrium within the apparatus, flows out through check valves 17 and 18. Purging of stagnated parts of the separator vessel 21 is effected by purge fluid injecting devices 12 and 13. The most suitable way is to run part of such purge fluid into the annular space between the concentric pipes 6 and 7 and to run the other part, together with the stripping fluid, countercurrent to the solids which flow out of the mouth 19 of the diplegless cyclone 4. In this invention the material flowing from the interior of the separator vessel 21 into the cyclone 4 though the mouth 19 consists of 0.1 to 20% of the total volume that flows along outlet pipe 11. The remaining material flows from inside the separator vessel by means of the annular space between the concentric pipes 6 and 7. In an alternative mode of operating this invention, whenever it is desirable a suspension of particles and reacted mixture can be bled through the lower mouth 19 of the diplegless cyclone 4. This helps a lot towards achieving greater efficiency in the gathering of solids within the separating vessel. In confining the gas stream from the separator vessel 21 to within the cyclone 4 any reentrainment of catalyst particles is prevented, this being the great problem in dealing with unconfined cyclones. Hydrocarbons bled off, representing from 3 to 20% of the outflow to the fractioning system, come from the separator vessel 21, together with the stripping fluid, the purging fluid and the catalyst particles, either out of the upper part of the diplegless cyclone 4 or in through the annular space between the concentric pipes 6 and 7. To carry out this other mode of operation, the apparatus represented in Figures 4 and 5 can be used. These Figures show details of assembly of the concentric pipes of Figure 1 (here referenced 40 and 41), where small windows 48 fitted into the conical stretch which joins the pipe 41 to pipe 40 help the bled fluid to flow to the inlet of the first stage cyclone. This operating alternative of the invention comes into play whenever the flow of fluid injected through the purge fluid injecting devices 12, 13 is small. This serves to show the biggest advantage of the process of this invention, namely the flexibility of the outlet control over purge and stripping fluids and some of the cracked hydrocarbon vapour that flows out from inside the separator vessel 21. This control can be effected with the aid of the solids discharge mouth 19 of the diplegless cyclone 4, or with the aid of the annular space between the concentric pipes 6 and 7 of said cyclone, when the unit is in operation. Two examples of suitable arrangements, out of the many possible for this invention, are shown in Figures 6 and 7, namely: (in Figure 6) the provision of a distributor 22 for the downward flow of particles separated in the diplegless cyclone 4; and (in Figure 7) a design of the same kind of cyclone provided with concentric pipes 26, 27 connecting it to the primary cyclone 8. From the foregoing, other easily perceived advantages of the invention are, for example:- (a) Since there is no leg to the cyclone 4 connected directly to the riser, most of the catalyst particles separated out from the reaction stream flow out directly through its open lower mouth 19 and fall in a smooth and diluted fashion into an environment saturated with purge and stripping fluids, down to the bottom of the separator vessel 21. Along this stretch a great amount of stripping of the catalyst is already taking place because the intraparticle transfer of mass is much favoured and this fact greatly reduces the need of stripping in the dense phase, as happens in the usual way of operating; it even does away with the need to inject fluid specifically for such purpose, thereby considerably simplifying the design and operation of such processes. (b) The ease of changing from one operating mode to another, with a return to conditions near to the usual ones in the traditional process, merely by regulating the flow rates of purge fluid so as to raise yields of gases and LPG, some of the hydrocarbons of which can be used as petrochemical feedstocks. This operating mode can be economically attractive according to the season. (c) The fact that it is easy to introduce the system of this invention into existing units with only minimum modifications to any equipment already installed; the only thing needed is that the size of the system must be in keeping with the capacity of the unit in which it is to be employed.

Claims: Apparatus for separating suspensions of catalyst particles and reacted mixture of hydrocarbons in catalytic cracking processes, comprising a diplegless cyclone (4) connected by means of concentric pipes (6, 7; 26, 27; 30, 31; 40, 41) directly to a primary cyclone (8), said diplegless cyclone (4) being connected to a riser (2) and having a lower opening at a mouth (19) and an upper opening at an annular space about the concentric pipes (6, 7; 30, 31; 40, 41), and wherein the respective openings communicate directly into a large volume separator vessel (21). Apparatus according to claim 1, wherein said cyclone (4) has in its lower mouth (19) a distributor (22) for better control over the downward flow of separated solid particles. Apparatus according to claim 1 or 2, wherein the said diplegless cyclone (4) has said concentric pipes (26, 27) built into the structure of the cyclone itself. Apparatus according to claims 1 or 2, wherein fitted into said concentric pipes (40, 41) are joined by a conical stretch into which small windows (48) are fitted. Apparatus according to any one of claims 1 to 4, wherein one or more purge fluid injecting devices (12, 13) are contained in the separator vessel. A catalytic cracking process for hydrocarbons comprising the following stages: (a) mixing the hydrocarbon feedstock with the suspension of catalyst particles in a catalytic cracking zone (1); (b) cracking said feedstock in a riser (2); (c) feeding the reacted suspension into a cyclone separator system to bring about separation of the gas phase from the particle phase, and leading a separated gas stream into a fractioning system out of an outlet pipe (11); (d) collecting the separated particle phase in a smaller diameter vessel (5) lying in the bottom part of a separator vessel (21) from where it flows into a regenerating zone; (e) purging stagnated parts of the separator vessel (21) by injecting a purge fluid through purge fluid injecting devices (12, 13); and (f) stripping catalyst in the separator vessel (21) by countercurrently injecting stripping fluid in the downward stream of catalyst; characterized in that said feed stage (c) of the reacted suspension takes place directly from the riser (2) to an unconfined cyclone device comprising a diplegless cyclone (4) having a lower opening at a mouth (19) and an upper opening at an annular space between concentric pipes (6,7; 30,31; 40,41; 26, 27), said openings communicating with the separator vessel (21); and in that in said stage (e) the purge flow through purge fluid injecting devices (12, 13) is adjusted so that all of the purged material, together with the stripping fluid, is able to flow from within the separator vessel (21) through said annular space between the concentric pipes (6,7; 30,31; 40,41; 26, 27). A process according to claim 6, wherein the purge flow through the purge fluid injecting devices (12, 13) is adjusted so that most of the purged material is able to flow from within the separator vessel (21) through said annular space between the concentric pipes (6,7; 30,31; 40,41; 26, 27), whilst all of the stripping fluid and a smaller part of the injected purge fluid, together with the stripped gas, flow countercurrently to the stream of solids, through the lower mouth (19) of the diplegless cyclone (4). A process according to claim 7, wherein the discharged gas which flows into the lower mouth (19) of the diplegless cyclone (4) comprises from 0.1 to 20% by volume of all the material that flows along the outlet pipe (11). A process according to claim 6, wherein part of stage (e) is brought about by bleeding off the particle suspension and the reacted mixture through said lower mouth (19) of said diplegless cyclone (4), and afterwards, flowing said bled off suspension together with the stripping fluid and some of the purging fluid through the annular space (49) between the concentric pipes (40, 41) and through small windows (48) in a conical stretch which connects such pipes. A process according to claim 9, wherein said bled material accounts for 3 to 20% by volume of the discharge into the fractioning system along said outlet pipe (11). A process according to any one of claims 6 to 10, wherein said stage (f) is carried out without injecting any stripping fluid. A process according to any one of claims 6 to 11, wherein any operational variations in the riser (2) are automatically offset by the lower mouth (19) of the cyclone (4) which opens into said separator vessel (21) which is large in size as compared with the cyclone device (4).

Inventors: The designation of the inventor has not yet been filed
Applicants: PETROLEO BRASILEIRO SA, PETROLEO BRASILEIRO S.A. - PETROBAS
Application year: 1994
Main classification: C10G11
IPC codes: B01J8, B04C9, C10G11, F27B15